Water treatment process

ABSTRACT

AN ELECTRICAL CURRENT SYSTEM IN CONJUCTION WITH LOW LEVEL CHELATION TREATMENT HAS BEEN FOUND TO ALLEVIATE PROBLEMS OF CORROSION AND PRECIPITATION OF UNSTABLE DISSOLVED SOLIDS AT BOTH LOW AND ELEVATED TEMPERATURES.

United States Patent 3,651,189 WATER TREATMENT PROCESS Harry T. Anderson, Clarendon Hills, 111., assignor to Swift & Company, Chicago, Ill. No Drawing. Filed Apr. 21, 1969, Ser. No. 818,052 Int. Cl. C23f 13/00 U.S. Cl. 204-147 7 Claims ABSTRACT OF THE DISCLOSURE An electrical current system in conjunction with low level chelation treatment has been found to alleviate problems of corrosion and precipitation of unstable dissolved solids at both low and elevated temperatures.

This invention relates to the stabilization of dissolved solids in water. It is particularly directed to the protection against corrosion of metallic structures or items immersed in mild to highly corrosive water. The invention concerns various water hardness problems which produce objectionable film formations, scale and water spotting on metallic items such as tin-containing cans, aluminum-containing cans and metal-covered glass jars that have been in contact with the water. In addition, it is also directed to the control of red water problems caused by high iron content aquifer structures and other similar systems.

The various problems involved in corrosion control and protection of metallic surfaces have been the subject of extensive research and investigation for many years. The most obvious and simple method for metal protection is to coat the exposed surfaces of the structure or articles with a paint, varnish or other coatings. Such a coating, which acts merely by physical isolation of the articles or structure from the corrosive medium, is wholly inadequate in cases where the medium is at all corrosive, since any minute defect or gap in the coating will initiate an attack which will expand approximately logarithmically with time. 1

Generally speaking, there are two well known methods used to attack the problem of metallic corrosion. In one process, known as passivation the exposed surfaces of the structure to be protected are coated with a composition so selected that certain constituents thereof will react chemically with the metal of the structure to yield reaction products protecting the surfaces against corrosion. Phosphatizing is a well known example of a passivation treatment and can be conducted by applying a coating to the metal surface by treatment with phosphoric acid which will react with the metal and form a very thin protective phosphate coating on the surface. Phosphatizing as well as other passivating treatments are usually inadequate in that they do not permanently control corrosion and in fact, the protection is very brief in many systems. The resulting coating is relatively thin, about 10 to microns and the bonding is not of a permanent nature. Moreover, such a coating, even though it is initially insoluble in the surrounding medium, gradually undergoes incipient hydrolysis so that the coating grows more, and more soluble with time. For one reason or another, passivating treat ments have not succeeded in adequately protecting immersed metal structures beyond brief periods although the time varies depending upon the system under control and its environment.

A more adequate method for metal corrosion prevention is a process known as cathodic protection and involves immersing in the surrounding corrosive medium one or more sacrificial or nonsacrificial elements spaced from the structure but connected in circuit therewith so as to act as anodes with respect to the structure which in turn ice acts as the cathode. In the intervening medium which is an electrolyte, a potential is created between the anodes and the structure. However, the extent to which the metal structure is actually protected in this way is measured by the potential of the electrolyte to the cathode at the cathode. This is determined by using a suitable half cell (such as a copper-copper sulphate) and a suitable direct current voltmeter and ammeter. Current must flow from the electrolyte to the cathode (structure) to insure protection of the structure.

Thus structures, such as underground metal piping in moist ground have been provided with a protective coating and simultaneously associated with cathodic protec tion. Such prior attempts have yielded satisfactory results in the case of underground structures. Cathodic protection, however, has not controlled red water problems in wells producing water with high iron content.

A related problem to the control of corrosion is spotting and film formation caused by salt precipitation from hard water. Both water hardness and corrosion produce objectionable film formations and water spots in various processing operations such as in canning. These films or spots dull or cloud the appearance of the tin plate and the lithographed surfaces of the cans. In canning operations, the surfaces can be dulled to such an extent that personnel are required to wipe each individual can to restore the surface to an acceptable appearance. However, wiping does not restore the surface to its original lustre. The use of personnel to wipe individual cans is economically prohibitive under todays conditions, yet if the films are permitted to remain, deterioration of the tin plate of the lithographed surfaces will continue. Damaged surfaces provide less protection against normal storage conditions and in adverse storage environments, loss of product becomes most certain.

In other aqueous systems, films on processed equipment cause deterioration of the metal. These film forming materials produce scale which restricts the flow of water and reduces heat transfer rates. This reduced heat transfer is particularly disadavntageous in equipment such as heat exchangers, cooling towers, condensers, etc. Severe chemical treatment is often required to restore flow of heat transfer rates and rock-like scale is often drilled out of heat exchanger tubes to restore heat transfer rates.

In regard to water hardness problems, sodium ion exchange units have been utilized to convert calcium salts to sodium salts which are more soluble. Hydrogen ion exchange resins have been devised to remove most cations, however, these are expensive treatments and in the case of a hydrogen ion exchange unit, some hazard exists inasmuch as the resin must be regenerated with an acid. In

addition, water from-such treatments is often very cor-' rosive and requires additional treatment. Other methods of reducing hardness and iron content of water include hot or cold lime softening, coagulation, flocculation, aeration, the use of oxidizing agents, retention tanks, filtering mechanisms or combinations of these systems. Generally speaking, however, these processes are expensive and oftentimes; impractical.

Objectionable film formation, scale and water spotting is not'restricted to warter hardness since corrosion also contributes to the problem. Generally, corrosion inhibtors and galvanic as well as impressed currents can generally control corrosion. However, galvanic or impressed current systems will not prevent film formations with either high or rather low hardness waters in many process systems. As a matter of fact, addition of calcium carbonate to lower the resistivity is desirable in low hard ness waters where impressed current systems are employed to control corrosion. Chelating agents and sequestering agents have been known to improve the stability of water hardness at low temperatures. The solubility of calcium complexes with chelating agents and sequestering agents decreases with increased water temperatures in process equipment up to a point that at high temperatures, chelating agents and sequestering agents have little efiect on the stability of water hardness. Further, large amounts of chelating and sequestering agents are required to stabilize water. Even in large amounts, none of these agents is capable of stabilizing water containing high quantities of ferric iron. Ferric iron and other water hardness materials will precipitate at temperatures far below the boiling point of water.

It is therefore as object of this invention to provide an improved method or means of protecting metallic surfaces against corrosion with particular reference to the surfaces of metallic structures immersed in heavily corrosive media such as brine, water of high and low total dissolved solids and hard water.

Another object of this invention is to provide a cathodic protection system which will be efiective in both acidic and alkaline medium.

Another object of this invention is to provide a system which 'will prevent objectionable film formation, scale and water spotting on items processed in either hot or cold aqueous systems.

Another object of this invention is to provide a system which will bring red water problems under control.

Additional objects, if not specifically set forth herein, will be readily apparent to those skilled in the art from a reading of the detailed description of the invention which follows.

Generally speaking, the objects of this invention are accomplished by the process of causing electrical current to flow through an aqueous medium so as to produce a substantially uniform and adequate cathode potential at all areas of the receptacle wall. In addition to the cathodic protection step, and in accordance with the invention, it has been discovered that by incorporating substances of certain types or classes, as described below, into the body of liquid to which the metal structure and/or items are exposed, one or more of the above objectives are accomplished. For the purposes of this invention, the term receptacle is meant to include tanks, pipes, etc. functioning as a restraining container for liquids. Non-limiting examples of restraining containers include cooking tanks, well structures, heat exchangers,

cooling towers, condensers, etc.

The present invention is designed to provide improvements in the procedure of cathodic protection, e.g. for attaining more complete or more uniform action with respect to the corroding areas, for reducing the number of anodes needed, for rendering the anode positions less critical, and for reducing the electrical power required; the improvements being especially significant under certain conditions such as a high hardness, high iron content, and/or high temperature water. The objects of the invention are accomplished by providing chelating or sequestering agents in an amount of about 1% to 15% of the stoichiometric calcium value in conjunction with the cathodic protection in such a manner that they both coact in a synergistic, mutually-enhancing manner leading to greatly improved results both in the efliciency of the protection achieved and in the economy of operation of the process.

Generally, in establishing the electrical circuit, one or more anodes are placed in contact with the liquid and the receptacle holding the liquid will function as the cathode. It is mentioned at this time that for the sake of brevity, the cathode will be defined as the wall of the tank, piper or other restraining means in the examples and in the description that follows. However, an equivalent embodiment would be the use of rods or sheets of metal placed sufliciently close to the walls of the receptable which would act as a suitable cathode-This embodiment would be used when the walls of the receptacle are generally poor or non-conductors of electrical current.

It has been found that proper distribution of direct current and a fractional percent of the calcium stoichiometric value of the chelating or sequestering agent can control corrosion, eliminate objectionable film formation, scale and water spotting and control red water problems with waters having low or high temperatures or even water that is subjected to a temperature gradient. At this point, it is mentioned that chelating agents such as ethylenediamine tetraacetic acid and sequestering agents such as sodium gluconate, sodium hexametalphosphate will improve the stability of water hardness at low temperatures. On the other hand, the ability of these agents to complex ions at high temperatures decreases sharply at elevated temperatures. This causes calcium and other ions to precipitate out onto the metal surface to a greater extent at the higher temperatures. In addition, none of these agents is capable of stabilizing water containing high ferric iron and it and other water hardness ions will precipitate at temperatures far below the boiling point of water. Further, none of these agents will arrest corrosion over an extended period of time and many fail to provide even brief corrosion control.

One of the surprising aspects of the invention is that, in coaction with the cathodic protection, the chelate or sequestering agent treatment need not be at or even near the stoichiometric level required for chelation. The advantage of this is obvious. A chelate added at about 5% of the calcium stoichiometric value will produce excellent results. Red water due to the high level of iron content requires the chelate at about 12% of the stoichiometric iron content. In this connection it has been found that a chelate in the range of about 1% to 15% of its calcium stoichiometric value will produce good results. It is be lieved that in the instant system, as it is applied to red water producing wells, ferrous iron is chelated and prevented from being converted to the ferric state. Structures, such as underground metal piping in moist ground have been provided, in prior art procedures, with a protective coating and simultaneously associated with spaced anode elements. Such prior attempts have yielded satisfactory results in case of underground structures but have generally failed completely when applied to structures immersed in NaCl brine solutions. However, by following the teachings of the instant invention, the impressed direct current-chelate addition system is capable of arresting corrosion in sodium chloride brine solutions (20% NaCl) and/or other extremely corrosive systems.

In designing the electrical circuit, the anode can be of any of the typical metals used in anode construction. It is preferred to use a highly silica-iron anode inasmuch as such material has a very low attrition rate. However, for purposes of this invention, any material capable of carrying a current is theoretically possible to serve as the anode. Additional factors that should be considered when designing the electrical circuit include the electrode profile, surface conditions, resistance value of electrode material, distance between electrodes, number of electrodes, electrode potentials and the geometry of electrode placement configuration, especially in cells with multiple anodes and/or cathodes.

Excessive anode potential levels should be avoided since too high a potential will produce hydrogen stress corrosion. Furthermore, in a canning operation, too high a potential will disbond the paint or varnish otf of the cans in a cooking or chilling tank. In this connection, in conducting the system in regard to canning operations, an anode potential ranging between one volt and ten volts, preferably four to seven volts, is utilized. The actual potential utilized will depend on the water hardness, resistance value and chlorine content. Generally speaking, if the chloride ion content is high, a relatively lower potential should be used so that free chlorine does not evolve. More particularly, in carrying out the process of this invention during a food cooking process, the steel tank walls will normally possess a potential of about .7 volt. It has been found that best results are accomplished when the copper-copper sulphate half cell at the tank wall is of a value of .85 volt or higher. This is in the case of steel structures and reading at galvanized structure surfaces should read near 1 to 1.1 volts. In some instances the difference between the potential at the tank wall can be as little as .60 volt but for an assurance value, at least .85 volt are utilized so as to be sure that there is a driving force difference at all points on the tank wall. In the canning operation, the anode potential is usually about 2.5 to 7 volts, yet on the other hand a potential of 20 volts or higher can be used if the anode is stationed such that the current density dissipates to a high degree prior to reaching the cans and walls of the tank. The anode also must not operate at a potential that will over-polarize the cathode.

In most cases, the pH of the Water does not prevent use of the impressed current and water of pH values ranging between pH 1 and pH of 11 have been treated in accordance with this invention.

The chelates and sequestering agents of this invention can be added by introduction in dry or solution form either to the body of liquid in the tank, or by proportionate feed to the water when the water is flowing, such as in aquifiers. Experience reveals that for most purposes, in high hardness waters, the amount of chelating agent or sequestering agent to be added will lie within the range of about 2 to 20 parts per million of water. This should be contrasted with requirements of 200 to 2,000 parts per million required when the chelate is used alone and not in conjunction with the cathodic protection system. Examples of suitable chelating and sequestering agents include sodium gluconate, sodium glucoheptonate, citric acid, sodium hexametaphosphate, phytic acid, ethylenediamine tetraacetic acid, diand tetra sodium salts of ethylenediamine tetraacetic acid, sodium salt of hydroxyethyl ethylenediamine triacetic acid, a sodium salt of dihydroxyethylglycinate and a sodium salt of nitrilotriacetic acid.

More in detail, the chelating and sequestering agents utilized in this invention are able to attach themselves to metallic ions and thereby deactivate the ion from further reaction and/or precipitation in the medium involved. Generally, the terms chelating agent and sequestering agent are used interchangably and should be so construed for purposes of this invention. Strictly speaking, however, when a metal ion combines with an electron donor, the resulting substance is said to be a complex, or coordination compound. If the substance which combines with the metal contains two or more donor groups so that a ring is formed, the resulting structure is said to be chelate and the donor is said to be a chelating agent. The chelating agents utilized in this invention are capable of stabilizing calcium and magnesium ions in an aqueous system. These agents are effective in concentrations far less than would be stoichiometrically required to form complexes with hardness cations.

Water-soluble chelating compounds are well known in the art and are represented by such compounds as the amino acids and derivatives, such as ethylenediamine tetraacetic acid or other polyalkylene, polyamine, polyacetic acid compounds, including the poly acids of the alkylol constituents of the polyamines. Other chelating compounds having active groups that will function adequately include chelates that possess carbonyl radicals, sulfonic acid radicals, amine radicals, phosphonic acid radicals, and the like.

The chelates and sequestering agents of the invention are used in varied dosages depending upon the media being treated. As little as one part per million will be effective in some media, and a range of 0.5 to 50 or 100 parts per million, up to about 200 parts per million for brine; however preferably above one part per million to 50 parts per million will generally suffice for most systems. Dosages in excess of 200 parts per million such as up to 500 parts per million may however, be required in a particular system where the dissolved solids are particularly severe. However, in the industrial cooking of canned food products, amounts up to 50 parts per million are more than adequated. It can thus be seen that these organic chelates are used in synergistic or trace amounts. Of course larger dosages may be used without detrimental effects but they are generally not necessary. The following examples are set forth as illustrated embodiments of the invention and are to be taken in any manner as limiting the scope of the invention which is defined by the appended claims.

EXAMPLE I Two stainless steel tanks of about 40 inches by 40 inches and 30 inches deep were used to check film formation control. The tanks were filled half-way with water having a pH of about 7.8 and container 8.1 grains of total hardness (as calcium carbonate). A few test tin cans were placed on racks submerged in the tanks and chelating and sequestering agents were added to the water while heat was applied. Treatment levels were at 1000, 100, 50, 20, 10, 5 and 2 ppm. using the following different chelating and sequestering agents: sodium gluconate, sodium glucoheptomate, citric acid, sodium hexametaphosphate, phytic acid, ethylenediamine tetraacetic acid, diand tetrasodium salts of ethylenediamine tetraacetic acid, pentasodium salt of diethylenetriamine pentaacetic acid, sodium salt of hydroxyethyl ethylenediamine triaacetic acid, and sodium salt of nitrilitriacetic acid. The lower levels were checked to determine if a threshold treatment would produce adequate results. All of these tests failed to produce adequate film formation control on the tin cans. Some of the EDTA type chelates produce better heat stability. However, all failed at normal cooking temperatures (250 F.).

EXAMPLE II Two magnesium anodes submerged in a tank of the dimensions of 40 inches by 40 inches and 30 inches deep were electrically insulated from the walls thereof and the same chelating and sequestering agents in the amounts set forth in Example I were added to the water. The magnesium anodes, having a higher potential than the iron (1.5 volts vs. .7 volt for steel), functioned as a sacrificial metal anode. The water had a hardness value of 8.1 grains, and it was found that chelating and sequestering agents in the range of 2 to 10 ppm. function adequately in controlling film formation, scale and water spotting on tin cans that had been submerged in the liquid for a period of 6 hours.

EXAMPLE III Since magnesium anodes are not compatible with a large number of waters, it was decided to use high silica iron anodes for the remainder of the tests. A steel tank having the dimensions of 40 inches by 40 inches and 30 inches deep was filled half full with water having a hardness of 8.1 grains. An impressed current system was employed using a portable test rectifier. A steel support rack was placed within the tank and supported several tin cans. The tin cans had been filled with food and possessed lithograph surfaces. Chelates as recited in Example I were added to the water system in amounts of their stoichiometric value and the results produced in the series of tests were excellent. There was no discoloration of tinplate or of the lithograph surfaces.

Perhaps it should be mentioned at this time that tin is normally cathodic to iron. If tinplate is damaged the base metal exposed is iron. As the iron corrodes, it becomes cathodic to the tin because the rust is of a lower potential than the tin. Thus more tin is sacrificed by virtue of becoming anodic to the rust resulting in exposure of more iron which is subject to rusting.

EXAMPLE IV This example relates to the problem of high iron content water and the system of this invention was set up in a municipal well pumphouse. A test water line was 7 used which employed a sacrificial zinc anodes. Chelating and sequestering agents as exemplified in Example I were checked at treatment levels previously listed. The well system was regulated fiow treatment system. A chelating agent was added to the flowing water nad samples of water were taken from the discharge end. Control samples were taken at the start of each test cycle. All control samples appeared clear at first but when heated to the boiling point for a couple of minutes produced cloudy, colored water before the boiling point was reached. Of the several chelating and sequestering agents tested, the disodium salt of ethylenediamine tetraacetic acid proved to be the most effective. For filtering tests, micron porosity millipore filter pads were used to filter control samples and treated water samples. Cold water samples produced clear millipore pads on both control and treated samples. Iron is in the soluble form as it comes from the well and is converted to the ferric state as it is exposed to the inner surface of corroding pipe. The iron content of the water coming from the well was from about .8 p.p.m. to 1.1 p.p.m. Total hardness was at 18 grains. A treatment of 2 p.p.m. with the disodium salt of ethylenediamine tetraacetic acid proved as effective as any higher treatment level. Chelating agents were added to the main water line 1500 feet from the well and it was found that 5 p.p.m. of the chelating agents were required to give adequate results.

EXAMPLE V A sodium chloride solution of 8 grain total hardness and pH of 7.8 was poured into an iron pail. An iron silicon anode was centrally placed in the water and tin cans were submerged and rested on a support. An impressed current in brine will release chlorine at an anode potential above about 3 volts. No objectionable release of chlorine was noticed when the potential was 2.7 volts and the potential was not raised above this. An impressed current at this potential will arrest corrosion for some time, however, the walls will eventually start to develop evidence of corrosion at the interface. Such a system operated for two weeks before corrosion was noted. Adding a treatment at 50 p.p.m. of the disodium salt of ethylenediamine tetraacetic acid prevented corrosion for several months. Repeated treatments at the same level restored the protection.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope hereof, and, therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In a process wherein water is in contact with ferrous metal subject to corrosion by said water, the method of reducing said corrosion which comprisesadding to said water a chelating or sequestering agent in an amount substantially less than required 'stoichiometrically to react completely with the alkaline earth metal ions but at least 1% of the stoichiometric'calcium value so as to be sufficient to prevent scale formation, positioning one or more anodes in contact with said water and impressing electrical current through the water from said anodes to said ferrous metal.

2. A process as in claim 1 wherein said ferrous metal is in the form of a food container and said food container is submerged beneath the water surface whereby the outside surface of said container is protected from film formation and scaling.

3. The process of claim 2 with the additional step that the water is heatedto a temperature of between about 160 F. and about 250 F.

4. The process of claim 3 wherein the metal food containers are selected from the group consisting of tin-containing cans, aluminum-containing cans and metal-covered glass jars.

5. The method of claim 1 wherein the agent added to the water is present in an amount of about 1%15% of the stoichiometric calcium value of the water.

6. The process of claim 1 wherein the agent is present in an amount of between about 1 p.p.m. to p.p.m. based on the weight of the water.

7. The process of claim 1 wherein the current is impressed at an anode potential between about 2.5 volts and about 7 volts.

References Cited UNITED STATES PATENTS 1,958,765 5/1934 Perkins 2o4 791 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R. 204-148 

